Method and Arrangement in a Wireless Communication System

ABSTRACT

The invention relates to a method and an arrangement in a transmitting node ( 560 ) for enabling a receiving node ( 540 ) to perform measurements on interference caused by transmissions from at least one transmission point ( 510, 520, 530 ) controlled by the transmitting node ( 560 ) on receptions at the receiving node ( 540 ). The transmitting and receiving nodes ( 560, 540 ) are comprised in a wireless communications system ( 500, 600, 700 ). The transmitting node determines ( 910 ) an interference measurement resource, IMR, comprising a set of Time-Frequency Resource Elements, TFREs, upon which the transmitting node is expected to transmit interference. The transmitting node then transmits ( 930 ) at least one interfering signal on said IMR as said interference. The at least one interfering signal comprises at least one of a desired signal, that is expected to be decoded or coherently measured upon by the receiving node ( 540 ) or another node ( 550 ) served by said transmitting node ( 560 ), and another signal, that is not expected to be decoded or coherently measured upon by any node ( 540, 550 ) served by said transmitting node ( 560 ). The desired signal is transmitted in place of the another signal as said at least one interfering signal on one or more TFREs of the IMR when the at least one transmission point is to transmit the desired signal to the receiving node ( 540 ) or to the another node ( 550 ). The another signal is transmitted on TFREs of said IMR where no desired signal is transmitted and the another signal is muted on TFRE&#39;s of said IMR where the desired signal is transmitted.

TECHNICAL FIELD

The present disclosure relates generally to methods and arrangements ina wireless communications system. In particular it relates tointerference measurements in a wireless communication system.

BACKGROUND

In a wireless communications system or cellular radio communicationssystem wireless devices and/or user equipments, also known as mobileterminals and/or wireless terminals, communicate via a Radio AccessNetwork (RAN) with one or more core networks. The user equipments may bemobile stations or user equipment units such as mobile telephones, alsoknown as “cellular” telephones, and laptops with wireless capability,e.g., mobile termination, and may thus be, for example, portable,pocket, hand-held, computer-included, or car-mounted mobile deviceswhich communicate voice and/or data via the radio access network. Awireless device may be any equipment being wirelessly connectable to aRAN for wireless communication.

The radio access network covers a geographical area which is dividedinto point coverage areas, traditionally denoted cells, with each pointcoverage area or cell being served by a base station, e.g., a Radio BaseStation (RBS), which in some networks is also called “eNB”, “eNodeB”,“NodeB” or “B node” and which in this document also is referred to as abase station or radio network node. A point coverage area is ageographical area where radio coverage is provided by a point, alsoreferred to as a “transmission point” and/or a “reception point”, whichis controlled by the radio base station or radio network node at a basestation site or radio network node site. A point coverage area is oftenalso denoted a cell, but the concept of a cell also has architecturalimplications and the transmission of certain reference signals andsystem information. More specifically, multiple point coverage areas mayjointly form a single logical cell sharing the same physical cell ID.However, in the following the notation of a “cell” is usedinterchangeably with “point coverage area” to have the meaning of thelatter. Moreover, a point, or “transmission point” and/or a “receptionpoint”, corresponds in the present disclosure to a set of antennascovering essentially the same geographical area in a similar manner.Thus, a point might correspond to one of the sectors at a site, e g abase station site, but it may also correspond to a site having one ormore antennas all intending to cover a similar geographical area. Often,different points represent different sites. Antennas correspond todifferent points when they are sufficiently geographically separatedand/or have antenna diagrams pointing in sufficiently differentdirections.

The radio network node communicates over an air interface or radiointerface with the user equipments within the range of the radio networknode. One radio network node may serve one or more cells via one or moreantennas operating on radio frequencies. The cells may be overlaid oneach other, e g as macro and pico cells having different coverage areas,or adjacent to each other, e g as so called sector cells where the cellsserved by the radio network node each cover a section of the total areaor range covered by the radio network node. The cells adjacent oroverlaid relative to each other may alternatively or additionally beserved by different or separate radio network nodes that may beco-located or geographically separated.

The one or more antennas controlled by the radio network node may belocated at the site of the radio network node or at antenna sites thatmay be geographically separated from each other and from the site of theradio network node. There may also be one or more antennas at eachantenna site. The one or more antennas at an antenna site may bearranged as an antenna array covering the same geographical area orarranged so that different antennas at the antenna site have differentgeographical coverage. An antenna array may also be co-located at oneantenna site with antennas that have different geographical coverage ascompared to the antenna array. In the subsequent discussion an antennaor antenna array covering a certain geographical area is referred to asa point, or transmission and/or reception point, or more specificallyfor the context of this disclosure as a Transmission Point (TP). In thiscontext multiple transmission points may share the same physical antennaelements, but could use different virtualizations, e.g., different beamdirections.

The communications, i e transmission and reception of signals betweenthe radio access network and a user equipment, may be performed over acommunication link or communication channel via one or more transmissionand/or reception points that may be controlled by the same or differentradio network nodes. A signal may thus, for example, be transmitted frommultiple antennas by being transmitted via one transmission point frommore than one antenna in an antenna array or by being transmitted viamore than one transmission point from one antenna at each transmissionpoint. The coupling between a transmitted signal and a correspondingreceived signal over the communication link may be modelled as aneffective channel comprising the radio propagation channel, antennagains, and any possible antenna virtualizations. Antenna virtualizationis obtained by precoding a signal so that it can be transmitted onmultiple physical antennas, possibly with different gains and phases.Link adaptation may be used to adapt transmission and reception over thecommunication link to the radio propagation conditions.

An antenna port is a “virtual” antenna, which is defined by an antennaport-specific reference signal. An antenna port is defined such that thechannel over which a symbol on the antenna port is conveyed can beinferred from the channel over which another symbol on the same antennaport is conveyed. The signal corresponding to an antenna port maypossibly be transmitted by several physical antennas, which may also begeographically distributed. In other words, an antenna port may bevirtualized over one or several transmission points. Conversely, onetransmission point may transmit one or several antenna ports.

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance is inparticular improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

The Long Term Evolution (LTE) standard is currently evolving withenhanced MIMO support. A core component in LTE is the support of MIMOantenna deployments and MIMO related techniques. A current workingassumption in LTE-Advanced, i e 3GPP Release-10, is the support of aneight-layer spatial multiplexing mode with possibly channel dependentprecoding. The spatial multiplexing mode is aimed for high data rates infavourable channel conditions. An illustration of the spatialmultiplexing mode is provided in FIG. 1. Therein, the transmittedsignal, represented by an information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W_(N) _(T) _(×r), which servesto distribute the transmit energy in a subspace of the N_(T)-dimensionalvector space, corresponding to N_(T) antenna ports. The precoder matrixis typically selected from a codebook of possible precoder matrices, andis typically indicated by means of a Precoder Matrix Indicator (PMI),which together with a Rank Indicator (RI) specifies a unique precodermatrix in the codebook. If the precoder matrix is confined to haveorthonormal columns, then the design of the codebook of precodermatrices corresponds to a Grassmannian subspace packing problem. The rsymbols in s each are part of a symbol stream, a so-called layer, and ris referred to as the rank or transmission rank. In this way, spatialmultiplexing is achieved since multiple symbols can be transmittedsimultaneously over the same Resource Element (RE) or Time-FrequencyResource Element (TFRE). The number of symbols r is typically adapted tosuit the current channel properties.

LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in thedownlink, and Discrete Fourier Transform (DFT) precoded OFDM in theuplink. The basic LTE physical resource can be seen as a time-frequencygrid, as illustrated in FIG. 2, where each time-frequency resourceelement (TFRE) corresponds to one subcarrier during one OFDM symbolinterval, on a particular antenna port. The resource allocation in LTEis described in terms of resource blocks, where a resource blockcorresponds to one slot in the time domain and 12 contiguous 15 kHzsubcarriers in the frequency domain. Two time-consecutive resourceblocks represent a resource block pair, which corresponds to the timeinterval upon which scheduling operates.

The received N_(R)×1 vector y_(n) for a certain resource element onsubcarrier n or, worded differently, data RE number n or TFRE number n,assuming no inter-cell interference, is modeled by

y _(n) =H _(n) W _(N) _(T) _(×r) s _(n) +e _(n)  (1)

where n denotes a transmission occasion in time and frequency, and e_(n)is a noise and interference vector obtained as realizations of a randomprocess. The precoder, or precoder matrix, for rank r, W_(N) _(T) _(×r),can be a wideband precoder, which may be constant over frequency, orfrequency selective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel H_(n), also denoted channel matrix, resultingin so-called channel dependent precoding. When based on User Equipment(UE) feedback, this is also commonly referred to as closed-loopprecoding and essentially strives for focusing the transmit energy intoa subspace which is strong in the sense of conveying much of thetransmitted energy to the UE or wireless device. In addition, theprecoder matrix may also be selected to strive for orthogonalizing thechannel, meaning that after proper linear equalization at the UE orwireless device, the inter-layer interference is reduced.

In closed-loop precoding, the UE or wireless device transmits, based onchannel measurements in the forward link, i e the downlink,recommendations to the radio network node or base station of a suitableprecoder to use. A single precoder that is supposed to cover a largebandwidth, so called wideband precoding, may be fed back. It may also bebeneficial to match the frequency variations of the channel and insteadfeed back a frequency-selective precoding report, e.g. severalprecoders, one per subband. This is an example of the more general caseof Channel State Information (CSI) feedback, which also encompassesfeeding back other entities or information than precoders to assist theradio network node or base station in subsequent transmissions to the UEor wireless device. Such other information may include Channel QualityIndicators (CQIs) as well as Rank Indicator (RI).

In Release 8 and 9 of LTE the CSI feedback is given in terms of atransmission rank indicator (RI), a precoder matrix indicator (PMI), andchannel quality indicator(s) (CQI). The CQI/RI/PMI report can bewideband or frequency selective depending on which reporting mode thatis configured. This means that for CSI feedback LTE has adopted animplicit CSI mechanism where a UE does not explicitly report e.g., thecomplex valued elements of a measured effective channel, but rather theUE recommends a transmission configuration for the measured effectivechannel. The recommended transmission configuration thus implicitlygives information about the underlying channel state.

The RI corresponds to a recommended number of streams that are to bespatially multiplexed and thus transmitted in parallel over theeffective channel. The PMI identifies a recommended precoder (in acodebook) for the transmission, which relates to the spatialcharacteristics of the effective channel. The CQI represents arecommended transport block size, i.e., code rate. There is thus arelation between a CQI and a Signal to Interference and Noise Ratio(SINR) of the spatial stream(s) over which the transport block istransmitted. Therefore, noise and interference estimates are importantquantities when estimating, for example, the CQI, which is typicallyestimated by the UE or wireless device and used for link adaptation andscheduling decisions at the radio network node or base station side.

The term e_(n) in (1) represents noise and interference in a TFRE and istypically characterized in terms of second order statistics such asvariance and correlation. The interference can be estimated in severalways. For example, estimates may be formed based on TFREs containingcell specific RS since s_(n) and W_(N) _(T) _(×r) are then known andH_(n) is given by the channel estimator. The interference may then beestimated as the residual noise and interference on the TFREs of theCell Specific Reference Signal (CRS), after the known CRS sequence hasbeen pre-subtracted, i.e., after the CRS has been cancelled. Anillustration of CRS, sometimes read out as Cell-specific ReferenceSymbols, for Rel-8 of LTE can be seen in FIG. 3. It is further notedthat the interference on TFREs with data that is scheduled for the UE inquestion can also be estimated as soon as the data symbols, s_(n) aredetected, since at that moment they can be regarded as known symbols.The latter interference can alternatively also be estimated based onsecond order statistics of the received signal and the signal intendedfor the UE of interest, thus possibly avoiding needing to decode thetransmission before estimating the interference term. Alternatively theinterference can be measured on TFREs where the desired signal, i e thesignal intended for the UE of interest, is muted, so the received signalcorresponds to interference only. This has the advantage that theinterference measurement may be more accurate and the UE processingbecomes trivial because no decoding or desired signal subtraction needto be performed.

In LTE Release-10, a new reference symbol sequence was introduced, theChannel State Information Reference Signal (CSI-RS), intended to be usedfor estimating channel state information. The CSI-RS provides severaladvantages over basing the CSI feedback on the CRS which were used, forthat purpose, in previous releases. Firstly, the CSI-RS is not used fordemodulation of the data signal, and thus does not require the samedensity. This means that the overhead of the CSI-RS is substantiallyless as compared to that of CRS. Secondly, CSI-RS provides a much moreflexible means to configure CSI feedback measurements: For example,which CSI-RS resource to measure on can be configured in a UE specificmanner. Moreover, antenna configurations larger than 4 antennas mustresort to CSI-RS for channel measurements, since the CRS is only definedfor at most 4 antennas.

A detailed example showing which resource elements within a resourceblock pair may potentially be occupied by UE-specific RS and CSI-RS isprovided in FIG. 4. In this example, the CSI-RS utilizes an orthogonalcover code of length two to overlay two antenna ports on two consecutiveREs. As seen, many different CSI-RS patterns are available. For the caseof 2 CSI-RS antenna ports, for example, there are 20 different patternswithin a subframe. The corresponding number of patterns is 10 and 5 for4 and 8 CSI-RS antenna ports, respectively.

A CSI-RS resource may be described as the pattern of resource elementson which a particular CSI-RS configuration is transmitted. One way ofdetermining a CSI-RS resource is by a combination of the parameters“resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which maybe configured by Radio Resource Control (RRC) signaling.

Based on a specified CSI-RS resource, that defines an effective channelfor the data transmission, and an interference measurementconfiguration, e.g. a muted CSI-RS resource, the UE can estimate theeffective channel and noise plus interference, and consequently alsodetermine which rank, precoder and transport format to recommend thatbest match the particular effective channel.

Furthermore, in order to improve system performance, for example byimproving the coverage of high data rates, improving the cell-edgethroughput and/or increasing system throughput, Coordinated Multipoint(CoMP) transmission and/or reception may be used in a wirelesscommunications system or radio access network. In particular, the goalis to distribute the user perceived performance more evenly in thenetwork by taking control of the interference in the system, either byreducing the interference and/or by better prediction of theinterference. To harvest the gains of introducing coordinatedtransmission or CoMP feedback it is essential that a radio network nodeor base station, e g an eNodeB, can accurately predict the performanceof a UE or wireless device for various coordinated transmissionhypotheses, in order to select an appropriate downlink assignment. Tothis end, accurate interference measurements at a terminal are a keyelement for CSI reporting targeting different transmission hypotheses.However, current state of the art solutions for interferencemeasurements are constrained by current standards and/or limitationsimposed by UE specific muting of data channels, making accurateinterference measurements difficult, in particular for CoMP systemsemploying dynamic point selection and/or joint transmission, where thetransmission point association to a UE varies dynamically in time.

Moreover, it is often beneficial for a scheduler in a radio network nodeor base station such as an eNodeB to receive CSI reports that are basedon a predictable and robust interference level. When a UE or wirelessdevice measures interference caused by other data transmissions, themeasured interference level will vary with the current traffic load andmoreover will see rapid power variations due to the so calledflash-light effect where dynamic precoding and/or beamforming ininterfering points cause rapid and often unpredictable interferencevariations. For CSI reporting, such variations typically degrade theoverall performance, since the measured interference oftenunderestimates the interference seen at the subsequent data transmissionallocation that is based on the CSI report. As a consequence, the radionetwork node or base station may have to reduce the data rate in thelink adaptation to avoid excessive retransmissions due to uncertaintiesin the reported CQI.

To improve the possibilities for UEs to perform accurate interferencemeasurements in a system zero-power (ZP) CSI-RS resources, also known asa muted CSI-RS have been introduced. The zero-power CSI-RS resources areconfigured just as regular CSI-RS resources, so that a UE knows that thedata transmission is mapped around those resources. The intent of thezero-power CSI-RS resources is to enable the network to mute thetransmission on the corresponding resources so as to boost the SINR of acorresponding non-zero power CSI-RS, possibly transmitted in a neighborcell/transmission point. For Rel-11 of LTE, a special zero-power CSI-RSthat a UE is mandated to use for measuring interference plus noise isunder discussion. As the name indicates, a UE can assume that the TPs ofinterest are not transmitting on the muted CSI-RS resource and thereceived power can therefore be used as a measure of the interferenceplus noise level. For the purpose of improved interference measurementsthe agreement in LTE Release 11 is that the network will be able toconfigure a UE to measure interference on a particular InterferenceMeasurement Resource (IMR) that identifies a particular set of TFREsthat is to be used for a corresponding interference measurement.

However, reserving resources for specific purposes such as interferencemeasurements reduces the resources available for data transmission. Insome system configurations such reserved resources may give rise to asignificant overhead, e g in the downlink when used to enable UEs toperform more reliable interference measurements.

Thus, there is a need for improving the efficiency in use of radioresources for reliably determining the interference that can be expectedwhen receiving a signal over a communication channel from a radio accessnetwork.

SUMMARY

It is therefore an object of at least some embodiments of the presentdisclosure to reduce the overhead incurred by interference measurementsin a wireless communications system.

According to a first embodiment of the present disclosure, this andother objects are achieved by a method in a transmitting node forenabling a receiving node to perform measurements on interference. Theinterference is caused by transmissions from at least one transmissionpoint controlled by the transmitting node on receptions at the receivingnode. The transmitting node and the receiving node are comprised in awireless communications system. The transmitting node determines aninterference measurement resource, IMR. The IMR comprises a set ofTime-Frequency Resource Elements, TFREs, upon which the transmittingnode is expected to transmit interference. The transmitting nodetransmits at least one interfering signal on the IMR as saidinterference. The at least one interfering signal comprises at least oneof a desired signal and another signal. The desired signal is a signalthat is expected to be decoded or coherently measured upon by thereceiving node or another node served by the transmitting node. Theanother signal is a signal that is not expected to be decoded orcoherently measured upon by any node served by the transmitting node.The desired signal is transmitted in place of the another signal as saidat least one interfering signal on one or more TFREs of the IMR when theat least one transmission point is to transmit the desired signal to thereceiving node or to the another node served by said transmitting node.The another signal is transmitted on TFREs of said IMR where no desiredsignal is transmitted and the another signal is muted on TFRE's of saidIMR where the desired signal is transmitted.

According to a second embodiment of the present disclosure, this andother objects objects are achieved by a transmitting node for enabling areceiving node to perform measurements on interference. The transmittingnode is configured to be connectable to at least one transmission pointfor communicating with the receiving node in the wireless communicationssystem. Receptions at the receiving node may be susceptible tointerference caused by transmissions from the at least one transmissionpoint.

The transmitting node comprises processing circuitry. The processingcircuitry is configured to determine an interference measurementresource, IMR. The IMR comprises a set of Time-Frequency ResourceElements, TFREs, upon which the transmitting node is expected totransmit interference. The processing circuitry is further configured totransmit, as said interference, at least one interfering signal on saidIMR. The at least one interfering signal comprises at least one of adesired signal and another signal. The desired signal is a signal thatis expected to be decoded or coherently measured upon by the receivingnode or another node served by the transmitting node. The another signalis a signal that is not expected to be decoded or coherently measuredupon by any node served by said transmitting node. The processingcircuitry further configured to transmit the desired signal in place ofthe another signal as said at least one interfering signal on one ormore TFREs of the IMR when the at least one transmission point is totransmit the desired signal to the receiving node or to the another nodeserved by said transmitting node. The processing circuitry is furtherconfigured to transmit the another signal on TFREs of said IMR where nodesired signal is transmitted and to mute the another signal on TFRE'sof said IMR where the desired signal is transmitted.

The above object is achieved since the transmission of a desired signalthat may be transmitted in place of the another signal as the at leastone interfering signal on an IMR enables the overhead, incurred byenabling more reliable interference measurements according to differentdesired interference compositions through the use of IMRs, to be reducedin that more radio resources are made available for data transmissionand other signaling intended to be decoded or coherently measured uponby receiving nodes in the wireless communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the transmissionstructure of the precoded spatial multiplexing mode in LTE.

FIG. 2 is a schematic diagram illustrating the LTE time-frequencyresource grid.

FIG. 3 is a schematic diagram illustrating cell-specific referencesignals.

FIG. 4 is a schematic diagram showing example layouts of referencesignals.

FIG. 5 is a schematic diagram illustrating a coordination cluster in awireless network.

FIG. 6 is a schematic diagram illustrating a coordination cluster in awireless network.

FIG. 7 is a schematic diagram illustrating a coordination cluster in awireless network.

FIG. 8 a is a schematic diagram illustrating a scenario in acoordination cluster in a wireless network.

FIG. 8 b is a schematic diagram illustrating a scenario in a wirelessnetwork.

FIGS. 9 a-9 d are flow charts illustrating methods according to someembodiments.

FIG. 10 a is a block diagram illustrating a network node according tosome embodiments.

FIG. 10 b is a block diagram illustrating details of a network nodeaccording to some embodiments.

FIG. 11 a is a block diagram illustrating a wireless device according tosome embodiments.

FIG. 11 b is a block diagram illustrating details of a wireless deviceaccording to some embodiments.

DETAILED DESCRIPTION

In this section, the invention will be illustrated in more detail bysome exemplary embodiments. It should be noted that these embodimentsare not mutually exclusive. Components from one embodiment may betacitly assumed to be present in another embodiment and it will beunderstood by a person skilled in the art how those components may beused in the other exemplary embodiments.

It should be noted that although terminology from 3rd GenerationPartnership Project (3GPP) LTE has been used in this disclosure toexemplify the invention, this should not be seen as limiting the scopeof the invention to only the aforementioned system. Other wirelesssystems, including Wideband Code Division Multiple Access (WCDMA),Worldwide Interoperability for Microwave Access (WiMax), Ultra MobileBroadband (UMB) and Global System for Mobile Communications (GSM)systems, may also benefit from exploiting the ideas covered within thisdisclosure.

Further, terminology such as eNodeB and UE should be considered asnon-limiting and does in particular not imply a certain hierarchicalrelation between the two; in general the term “eNodeB” or base stationcould be considered as a first device, first node or transmitting nodeand the term “UE” could be considered as a second device, second node orreceiving node, and these two devices communicate with each other over aradio channel that may be of various types, for example amultiple-input-multiple-output, “MIMO” channel. Herein, we also focus onwireless transmissions in the downlink, i e from the eNodeB to the UE,but the teachings of the embodiments described herein are equallyapplicable in the uplink, i e from the UE to the eNodeB. Thus, in suchembodiments the receiving node may be the eNodeB or base station and thetransmitting node may be the UE.

Generally, in systems with uncoordinated scheduling of downlinktransmissions, the UE may effectively measure interference observed fromother TPs, or other cells, when computing a recommendation forinterference level and use this in an upcoming data transmission. Suchinterference measurements are typically performed by analyzing theresidual interference on CRS resources (after the UE subtracts theimpact of the CRS signal).

However, in the present disclosure, the inventors have realized that insome situations, when it is important to obtain an accurate interferencemeasurement on a transmission resource, such as Time-Frequency ResourceElements (TFREs) of a radio transmission interface in a wirelesscommunication system, such measurements may not be easily obtainable.This may for example be the situation when traffic load is low in asystem and it is desired, e g for scheduling purposes, to determine howinterference on a specific transmission resource would impact schedulingof a data transmission to a receiving node such as a UE. Anothersituation when it may be difficult to obtain an adequate measuredinterference level is when the transmission scheme of an interferingtransmission point varies rapidly in an on-off way, e g from subframe tosubframe. In such situations, i e when the interference measurementswill not adequately reflect the interference situation, the CSIreporting for link adaptation and/or Coordinated Multipoint (CoMP)transmission will become corrupted. The term CoMP is sometimesunderstood to imply that different transmission points have differentgeographical locations. However, for the purposes of embodiments of thisdisclosure, the coordinated transmission aspect is relevant also forsituations where transmission points involved in coordinatedtransmission have the same geographical location. For example, multipletransmission points may in this context share the same physical antennaelements, but could use different virtualizations, e.g., different beamdirections, as mentioned in the earlier discussion about Transmissionpoints herein.

As mentioned above, interference measurement resources (IMRs), areadopted by the LTE standard to enable the network to better control theinterference measurements in the UEs. By muting a particular set oftransmission points on a corresponding IMR a UE will only measure theresidual interference caused by any non-muted transmission point in thevicinity.

To enable appropriate interference measurements by a receiving node inthe above exemplified and similar situations the inventors propose inthis disclosure to have at least some transmission point, orvirtualization thereof, whose interference is desired in a particularIMR, to actively transmit an interfering signal on the time-frequencyresource elements of the particular IMR. The interfering signal may be asignal that is independent of any data transmission, controltransmission or reference signal transmission to any node, e g UE orwireless device, or a desired signal, for example a data signal, that isto be transmitted from the transmission point and that can take theplace of the interfering signal on at least some TFREs of the particularIMR where the signal that is independent of any data transmission,control transmission or reference signal transmission would otherwise betransmitted. The receiving node may be expected by the network tomeasure interference on the IMR. This expectation may be implicit, forexample selecting an IMR where the receiving node is known to measure oninterference or explicit, for example by instructing the receiving nodeto perform interference measurements on the IMR. The interfering signalthat is independent of any data transmission, control transmission orreference signal transmission is independent in that it is not expectedto be decoded or coherently measured upon by any node served by thetransmitting node that controls the transmission point from which theinterfering signal is transmitted. The desired signal is desired in thatit is expected to be decoded or coherently measured upon by any nodesthat may be configured to receive transmissions from the transmissionpoint.

For the purpose of this disclosure an IMR is to be regarded in a widercontext than currently adopted in 3GPP. For example, an IMR should beregarded only as a set of time frequency resource elements that a UE isexpected, or will likely, estimate interference upon. For example, mostimplementations of Rel-8 to 10 LTE terminals measure interference on theTFREs associated with the cell specific reference signal (CRS), by firstcanceling the known CRS sequence. Hence, even though it is not mandatedby the standard that a Rel 8-10 UE shall measure interference on the CRSresource elements, this is the de facto standard. Hence, in thefollowing discussion also resource elements associated with a specificCRS configuration, i.e., a specific CRS shift and a specific number ofCRS ports, are considered an IMR. Similarly, it should be understoodthat an IMR may also contain other desired signals intended for decodingor reference for a UE, in which case a UE is expected to cancel theimpact of any such other desired signals, prior to performing theinterference measurement. “Desired signal” in this context means asignal intended for reception by the receiving node, e g UE or wirelessdevice. The interference measurement resource comprises a set ofresource elements in which one or more signals assumed to be interferingwith the desired signal are received. A reference signal resourcecomprises a set of resource elements in which one or more referencesignals corresponding to a desired signal are received. In particularembodiments the reference signal resource is a CSI-RS resource. However,the reference signal (RS) resource may be any other type of RS resourcewhich may be used to estimate a desired signal, e.g. a CRS resource.

To illustrate the teachings of this disclosure in more detail, somesuggested embodiments will now be discussed in a coordinatedtransmission or CoMP scenario in a wireless communication system.However the reference to CoMP in the following discussion of thisdisclosure is not to be understood as limiting for the applicability ofthe teachings herein. The teachings herein are equally applicable in anywireless communication system when there is a need to reliably determinethe interference that can be expected when receiving a signal over acommunication channel from a radio access network.

CoMP transmission and reception refers to a system where thetransmission and/or reception at multiple, geographically separatedantenna sites is coordinated in order to improve system performance.More specifically, CoMP refers to coordination of antenna arrays thathave different geographical coverage areas. The coordination betweenpoints can either be distributed, by means of direct communicationbetween the different sites, or by means of a central coordinating node.A further coordination possibility is a “floating cluster” where eachtransmission point is connected to, and coordinates, a certain set ofneighbors (e.g. two neighbors). A set of points that perform coordinatedtransmission and/or reception is referred to as a CoMP coordinationcluster, a coordination cluster, or simply as a cluster in thefollowing.

CoMP operation targets many different deployments, includingcoordination between sites and sectors in cellular macro deployments, aswell as different configurations of Heterogeneous deployments, where forinstance a macro node coordinates the transmission with pico nodeswithin the macro coverage area. In FIGS. 5-7 examples of wirelesscommunications network deployments with CoMP coordination clusterscomprising three transmission points, denoted TP1, TP2 and TP3 areshown.

There are many different CoMP transmission schemes that are considered;for example:

Dynamic Point Blanking where multiple transmission points coordinate thetransmission so that neighboring transmission points may mute thetransmissions on the time-frequency resources (TFREs) that are allocatedto UEs that experience significant interference.

Coordinated Beamforming where the TPs coordinate the transmissions inthe spatial domain by beamforming the transmission power in such a waythat the interference to UEs served by neighboring TPs is suppressed.

Dynamic Point Selection (DPS) where the data transmission to a UE mayswitch dynamically (in time and frequency) between differenttransmission points, so that the transmission points are fully utilized.

Joint Transmission where the signal to a UE is simultaneouslytransmitted from multiple TPs on the same time/frequency resource. Theaim of joint transmission is to increase the received signal powerand/or reduce the received interference, if the cooperating TPsotherwise would serve some other UEs without taking the UE subject toJoint Transmission (JT) into consideration.

A common denominator for the CoMP transmission schemes is that thenetwork needs CSI information not only for the serving TP, but also forthe channels linking the neighboring TPs to a terminal or UE. By, forexample, configuring a unique CSI-RS resource per TP, a UE can resolvethe effective channels for each TP by measurements on the correspondingCSI-RS. Note that the UE is likely unaware of the physical presence of aparticular TP, it is only configured to measure on a particular CSI-RSresource, without knowing of any association between the CSI-RS resourceand a TP.

Several different types of CoMP feedback are possible. Most alternativesare based on per CSI-RS resource feedback, possibly with CQI aggregationof multiple CSI-RS resources, and also possibly with some sort ofco-phasing information between CSI-RS resources. The following is anon-exhaustive list of relevant alternatives (note that a combination ofany of these alternatives is also possible):

Per CSI-RS resource feedback corresponds to separate reporting ofchannel state information (CSI) for each of a set of CSI-RS resources.Such a CSI report may, for example, comprise one or more of a PrecoderMatrix Indicator (PMI), Rank Indicator (RI), and/or Channel QualityIndicator (CQI), which represent a recommended configuration for ahypothetical downlink transmission over the same antennas used for theassociated CSI-RS, or the RS used for the channel measurement. Moregenerally, the recommended transmission should be mapped to physicalantennas in the same way as the reference symbols used for the CSIchannel measurement.

Typically there is a one-to-one mapping between a CSI-RS and a TP, inwhich case per CSI-RS resource feedback corresponds to per-TP feedback;that is, a separate PMI/RI/CQI is reported for each TP. Note that therecould be interdependencies between the CSI reports; for example, theycould be constrained to have the same RI. Interdependencies between CSIreports have many advantages, such as; reduced search space when the UEcomputes feedback, reduced feedback overhead, and in the case of reuseof RI there is a reduced need to perform rank override at the eNodeB.

The considered CSI-RS resources may be configured by the eNodeB as theCoMP Measurement Set. In the example shown in FIG. 5, differentmeasurement sets may be configured for wireless devices 540 and 550. Forexample, the measurement set for wireless device 540 may consist ofCSI-RS resources transmitted by TP1 510 and TP2 520, since these pointsmay be suitable for transmission to device 540. The measurement set forwireless device 550 may instead be configured to consist of CSI-RSresources transmitted by TP2 520 and TP3 530. The wireless devices willreport CSI information for the transmission points corresponding totheir respective measurement sets, thereby enabling the network to e.g.select the most appropriate transmission point for each device.

Aggregate feedback corresponds to a CSI report for a channel thatcorresponds to an aggregation of multiple CSI-RS. For example, a jointPMI/RI/CQI can be recommended for a joint transmission over all antennasassociated with the multiple CSI-RS.

A joint search may however be too computationally demanding for the UE,and a simplified form of aggregation is to evaluate an aggregate CQIwhich are combined with per CSI-RS resource PMIs, which should typicallyall be of the same rank corresponding to the aggregated CQI or CQIs.Such a scheme also has the advantage that the aggregated feedback mayshare much information with a per CSI-RS resource feedback. This isbeneficial, because many CoMP transmission schemes require per CSI-RSresource feedback, and to enable eNodeB flexibility in dynamicallyselecting CoMP scheme, aggregated feedback would typically betransmitted in parallel with per CSI-RS resource feedback. To supportcoherent joint transmission, such per CSI-RS resource PMIs can beaugmented with co-phasing information enabling the eNodeB to rotate theper CSI-RS resource PMIs so that the signals coherently combine at thereceiver.

For efficient CoMP or coordinated transmission operation it is equallyimportant to capture appropriate interference assumptions whendetermining the CQIs as it is to capture the appropriate receiveddesired signal. Within a coordination cluster an eNodeB may to a largeextent control which TPs that interfere a particular UE or wirelessdevice in any particular TFRE. Hence, there will be multipleinterference hypotheses depending on which TPs are transmitting data toother terminals, such as other UEs or wireless devices.

This means that, by controlling which TPs are transmitting data to otherUEs or wireless devices, the network may control the interference seenby the particular UE or wireless device on an IMR. For example, bymuting all TPs within a coordination cluster on the IMR, the particularUE or wireless device will effectively measure the inter CoMP clusterinterference. In the example shown in FIG. 5, this would correspond tomuting TP1 510, TP2 520 and TP3 530 in the TFREs associated with theIMR. However, it is essential that an eNodeB can accurately evaluate theperformance of a UE given different CoMP transmissionhypotheses—otherwise the dynamic coordination becomes meaningless. Thusthe system need to be able to track/estimate also differentintra-cluster interference levels corresponding to differenttransmission and blanking hypotheses. It has therefore been proposed toallow configuration of multiple distinct IMRs, wherein the network isresponsible for realizing different relevant intra-cluster and/orinter-cluster interference hypotheses in the different IMRs, e.g., bymuting the data transmissions accordingly on different transmissionpoints, and that a UE should be able to perform multiple interferencemeasurements, corresponding to different intra-cluster interferencehypotheses, by means of configuring multiple IMRs; thus enabling CSI orCQI reporting for the different interference hypotheses. Hence, byassociating a particular reported CSI or CQI with a particular IMR therelevant CSIs or CQIs can be made available to the network for effectivescheduling.

The network would thus be responsible for configuring the transmissionsso that the interference measured on the different IMRs corresponds tothe desired interference hypotheses; that is, for each IMR a set oftransmission points will be muted, and intra-cluster interference onlyfrom the remaining coordinated (and uncoordinated) transmission pointswill be present on the IMR. In the state-of-the-art solutions, the datatransmission on a specific transmission point will thus not be muted (orsimilarly muted) on the IMRs where interference from the transmissionpoint should be present (or absent).

However, in LTE, muting is configured by means of zero power CSI-RS,which is configured UE specifically. Thus, muting is a UE specificproperty, rather than a transmission point specific property. Thisdifference does not have any practical implications when the UE isallocated and served by a single transmission point. However, in systemsoperating e.g. with dynamic point selection (DPS) and/or jointtransmission (JT), where the transmission to a specific UE involves, orchanges between, multiple transmission points, there will be a mismatchbetween a configured muting pattern specific to the UE, and one of thepotentially different targeted muting patterns of two different involvedtransmission points. In the state-of-the art solution, a UE, candidatefor DPS/JT allocations, therefore would, typically, be configured to bemuted on the union of the muting patterns for the two TPs. However, thiswill lead to an underestimation of the interference levels on thecorresponding IMRs, since whenever the specific terminal is allocatedthe involved TPs will be muted also on IMRs where interference from theTP should be present.

Moreover, by realizing the different intra-cluster interferencehypotheses by means of having an IMR excited by intra-cluster datatransmissions the interference measurements will be impacted by thecurrent traffic load in the system; that is, if there in a specificinstance are no data for UEs that are candidates for transmissions froma particular TP, then there will be no data transmission that cancollide with the IMR. Traditionally this is beneficial since themeasured interference level will better reflect the typical, orexpected, interference. However, when a coordinated cluster oftransmission points are jointly (e.g., centrally) scheduled, thescheduler determines, and thus knows, the specific allocation on thecoordinated transmission points—that is, the scheduler evaluates the twohypotheses where there is interference from a TP and when there is not.Hence, if the interference measurements including intra-clusterinterference become biased by the load within the cluster, it will bemore challenging for an eNodeB to accurately evaluate the performance ofa UE for the hypothesis that there is interference present from aparticular TP. This problem will be increasingly pronounced the lowertraffic load there is within the cluster.

Similarly, if dynamic point blanking is applied, interferencemeasurements will be affected by rapidly varying on-off behavior oftransmission points that are dynamically blanked, i.e. muted, on certainsubframes. Since the UE is unaware of potential blanking on neighboringtransmission points it cannot choose to exclude interferencemeasurements on these time-frequency resources. The corresponding CSIreport will consequently not represent the desired interferencehypothesis.

FIG. 5 illustrates an example wireless communications system 500 inwhich various embodiments of the invention may be implemented. The threetransmission points 510, 520 and 530 form a CoMP coordination cluster.In the following, for purposes of illustration and not limitation, itwill be assumed that the communications system 500 is an LTE system.Transmission points 510, 520 and 530 are remote radio units (RRU:s),controlled by eNodeB 560. In an alternative scenario (not shown), thetransmission points could be controlled by separate eNodeBs. It shouldbe appreciated that, generally speaking, each network node, e.g. eNodeB,may control one or more transmission points, which may either bephysically co-located with the network node, or geographicallydistributed. In the scenario shown in FIG. 5, it is assumed that thetransmission points 510, 520 and 530 are connected to eNodeB 560, e.g.by optical cable or a point-to-point microwave connection. In the casewhere some or all of the transmission points forming the cluster arecontrolled by different eNodeBs, those eNodeBs would be assumed to beconnected with each other e.g. by means of a transport network, to beable to exchange information for possible coordination of transmissionand reception.

Consider for example a dynamic point blanking scheme, where there are atleast two relevant interference hypotheses for a particular UE: In oneinterference hypothesis the UE sees no interference from the coordinatedneighboring transmission point; and in the other hypothesis the UE seesinterference from the neighboring point. To enable the network toeffectively determine whether or not a TP should be muted, the UE canreport two, or generally multiple, CSIs or CQIs corresponding to thedifferent interference hypotheses. Continuing the example of FIG. 5,assume that the wireless device 540 is configured to measure CSI fromTP1 510. However, TP2 520 may potentially interfere with a transmissionfrom TP1 510, depending on how the network schedules the transmission.Thus, the network may configure the device 540 for measuring the CSI-RStransmitted by TP1 for two interference hypotheses, the first one beingthat TP2 is silent, and the other one that TP2 is transmitting aninterfering signal.

This situation is further illustrated in the example of FIG. 8 a.Therein, a UE or wireless device 540 has been configured withInterference Measurement Resources (IMRs) 815 comprising IMR1 850, IMR2855, IMR3 860 and IMR4 862 by the network, e g in RRC signalling. EachIMR 850, 855, 860, 862 comprises at least one TFRE upon which the UE 540is expected to perform measurements on interference. By configuring thecorresponding resources in time t and frequency f 810 at transmissionpoint TP1 510 and 805 at transmission point TP2 520 so that thetransmission points TP1 510 and TP2 520 in various combinations transmitor does not transmit interfering signals on the different IMRs, the UE540 is enabled to measure interference according to differentinterference hypotheses 875, 880, 885 and 887 as shown in table 870 inFIG. 8 a. From table 870 it can be seen that TP1 510 has been configuredby the network to transmit an interfering signal on IMR2 840 and IMR3845, and that from TP1 510 no interfering signal is transmitted on IMR1835 and IMR4 847. Further, it can be seen from table 870 that TP2 520has been configured by the network to transmit an interfering signal onIMR1 820 and IMR3 830, and that from TP2 520 no interfering signal istransmitted on IMR2 825 and IMR4 832. In this example, the interferingsignals are signals that are not expected to be decoded or coherentlymeasured upon by the UE 540 or any other node served by the transmittingnode that controls the transmission point from which the interferingsignal is transmitted. Further, in the example in FIG. 8 a, themeasurements performed on IMR1, IMR2 and IMR3 correspond to differentintra-cluster interference hypotheses H1 875, H2 880 and H3 885, whereasthe measurements performed on IMR4 corresponds to inter-clusterinterference hypothesis H4 887.

The interfering signal is not constrained by the configurations ofspecific UEs, but can be transmitted to match any interferencecomposition of choice. For example, if all UEs within the cluster areconfigured to receive data transmissions that are muted on the union ofthe present IMRs, then there will be no data (or control) transmitted onany IMR. Thus, the interference measured on the IMRs may in the aboveexample be unaffected by the intra-cluster data transmissions and thecomposition of the measured intra-cluster interference can be freelycomposed by the interfering signal(s).

In particular, an aspect is that interference from a particulartransmission point (TP) is always present on all IMRs that theparticular TP is expected to cause interference on. Otherwise theinterference measurements on that IMR will not reflect the intendedinterference hypothesis. However, if the PDSCH of all UEs of a servingnode are muted on all IMRs in a coordinated cluster this amounts to asignificant overhead in the downlink.

The inventor has realized that in some situations it may be desired toincrease the radio resources available for transmission of desiredsignals, such as data transmissions intended to be decoded by receivingnodes, or other desired signals intended to be coherently measured uponby receiving nodes, such as reference signals. FIG. 8 b illustrates ascenario where the interfering signal transmitted on IMR1 821 is adesired signal that is expected to be decoded or coherently measuredupon by the receiving node 540 or another node 550 served by thetransmitting node 560. The receiving node has been correspondinglyconfigured to expect that a desired signal may be sent on the TFREs ofIMR1 851. This transmitted desired signal may further be used byreceiving nodes in the wireless communication system, e g the receivingnode 540 or the another node 550 for measuring interference according tointerference hypothesis H1 875, as shown in table 871. As is furtherillustrated in FIG. 8 b, the interfering signal transmitted on a firstpart of the TFREs of IMR3 831, as indicated by horizontal stripes in theupper part of IMR3 831, is also a desired signal that is expected to bedecoded or coherently measured upon by the receiving node 540 or anothernode 550 served by the transmitting node 560. The receiving node hasbeen correspondingly configured to expect that a desired signal may besent on the TFREs of IMR3 861. This transmitted desired signal mayfurther be used by receiving nodes in the wireless communication system,e g the receiving node 540 or the another node 550 for measuringinterference according to interference hypothesis H3 885, as shown intable 871, by measuring on a first part of IMR3, indicated as IMR3 upperin table 871. Interference according to interference hypothesis H3 885may also be measured on a second part of IMR3, indicated as IMR3 lowerin table 871. On the TFREs of this part of IMR3 the interfering signaltransmitted by TP2 is another signal that is not expected to be decodedor coherently measured upon by the receiving node 540 or any other nodeserved by the transmitting node 560 that controls the transmission pointfrom which the interfering signal is transmitted. As this signal is notexpected to be decoded or coherently measured upon, the receiving node540 is configured not to expect any desired signal to be sent on theseTFREs, as indicated by lower part of IMR3 861 being blank. Thus, IMR3upper and IMR3 lower provides signals for measurement according to thesame Interference hypothesis. The difference is only that for IMR3 upperTP2 sends a desired signal and TP1 sends the another signal whereas forIMR3 lower TP1 and TP2 both send the another signal. A receiving node, eg a UE that may switch between TP2 and TP1 needs to measure differentinterference hypotheses. However, a receiving node such as a UE that isonly connected to TP 2 all the time need not perform such measurements,and can then be configured to expect that a desired signal such as aPDSCH data transmission may come on all the IMRs where TP2 is configuredto transmit, i e in this case on IMR1 851 and IMR3 861.

It should be noted that the illustrations given in FIGS. 8 a and 8 b aresimplified to facilitate understanding. In practice, an IMR resourcetypically may comprise TFREs that are distributed over substantially theentire frequency bandwidth, for example with 4 TFREs in each resourceblock along the bandwidth. A data transmission to a receiving node istypically scheduled per resource block. When a desired signal, such as adata transmission, is transmitted on TFREs of an IMR as indicated in theexample in FIG. 8 b, the transmission typically overlaps the TFREs inthe resource block where the data transmission occurs. In FIG. 8 b onlythe TFREs where this overlap occurs are illustrated. According to thisreasoning, IMR3 831 shows TFREs that belong two different resourceblocks. Only the TFREs in the upper part of IMR3 belong to resourceblocks where transmission of a desired signal occurs in such a way thatthe transmission overlaps the TFREs of the IMR3.

In short the illustrated embodiment of the invention involves dynamicalternation of a desired signal intended to be decoded or coherentlymeasured upon by a receiving node such as a UE and another signal notintended to be decoded or coherently measured upon by any receiving nodeon an interference measurement resource (IMR) which the network intendsto expose to a particular interference composition. Examples of desiredsignals may be data signals transmitted on the Physical Downlink SharedChannel (PDSCH). By letting the PDSCH from a particular TP betransmitted also in IMRs where interference should be present from theparticular TP, the overhead of the PDSCH can be decreased, e g ascompared to the case that only the above mentioned another signal istransmitted as interference signal on these particular IMRs, and bytransmitting the another signal in the particular IMRs whenever/wherevera PDSCH is not present it is guaranteed that the correct interferencecomposition is present on the particular IMR.

For UEs that are not candidates for transmissions involving other TPsthan the serving TP it is sufficient to configure these UEs with asemi-static configuration of a muting pattern, e.g., a zero-power CSI-RSconfiguration, that covers the IMRs where the particular TP should bemuted, whereas one or multiple IMRs where the TP should haveinterference present are not muted. Whenever the TP has a desired signalto transmit, such as a PDSCH transmission the data signal can take fulluse of all the resources also of the non-muted IMRs, and simultaneouslycause the desired interference on this set of IMRs. However, when the TPdoes not have a desired signal such as a data signal (PDSCH) to transmiton resources defined by the IMRs that should have interference present,then a separate interference signal in form of the above mentionedanother signal may be transmitted on those TFREs that are lacking thetransmission of a desired signal. Thereby the TP can guarantee thatinterference is always present on a selected set of IMRs, but stillutilize these IMRs for e g data transmissions when such are present.

In other words, by in this way enabling desired signals such as datatransmissions to be scheduled so that they overlap TFREs of an IMR, theoverhead incurred by enabling more reliable interference measurementsaccording to different desired interference compositions through the useof IMRs may be substantially reduced in that more radio resources aremade available for data transmission and other signaling intended to bedecoded or coherently measured upon by receiving nodes in the wirelesscommunication system. The embodiments of this disclosure achieve thatthe correct interference composition can be guaranteed to be present inan IMR while the PDSCH overhead is decreased, and thereby downlinkthroughput is increased

The above technique can also be used for UEs engaged in dynamic pointselection and/or joint transmission if the UE can be dynamicallyconfigured with a muting pattern, e.g. a ZP CSI-RS configuration, suchthat the muting pattern is always matched with the node/nodes from whichthe PDSCH transmission originates.

More generally, some embodiments provide a method in a transmitting nodefor enabling a receiving node to perform measurements on interference.The transmitting node enables the receiving node to perform measurementson interference by transmitting an interfering signal, as will now bedescribed with reference to FIGS. 5-7 and the flowcharts of FIGS. 9 a-9d. As a particular example, the transmitting node may be thetransmitting node, e.g. eNodeB, 560 in FIG. 5 controlling TP1-TP3, whichare transmission points or remote radio heads. In an alternativescenario, such as that shown in FIG. 6, the transmitting node may be atransmitting node or an eNodeB with three sector antennas whichcorrespond to transmission points TP1-TP3, forming a CoMP cluster 600wherein a receiving node 540 is located. In yet another scenario, asshown in FIG. 7, TP1-TP3 may form a CoMP cluster 700 wherein a receivingnode 540 is located, and the transmitting node may either be the eNodeBcontrolling TP1 and TP3, or the eNodeB controlling TP2, and serving picocell 720.

The interference is caused by transmissions from at least onetransmission point 510, 520, 530 controlled by the transmitting node 560on receptions at the receiving node 540. The transmitting and receivingnodes 560, 540 are comprised in a wireless communications system 500,600, 700. The transmissions from different transmission points 510, 520,530 may be coordinated in order to control interference and/or improvereceived signal quality in the wireless communications system 500, 600,700. In some embodiments the wireless communications system may beconfigured to apply Coordinated Multipoint Transmission.

A first embodiment of the method is shown in FIG. 9 a, wherein:

In step 910 the transmitting node 560 determines an interferencemeasurement resource, IMR. The IMR comprises a set of Time-FrequencyResource Elements, TFREs, upon which the transmitting node is expectedto transmit interference. According to some embodiments, the IMR maycomprise or be confined to TFREs upon which no signal that is expectedto be decoded or coherently measured upon by any node 540, 550 served bythe transmitting node 560 is pre-scribed to be transmitted. In otherwords according to these embodiments, a desired signal, that is expectedto be decoded or coherently measured upon by at least one of thereceiving nodes 540, 550 served by said transmitting node 560, is notpre-scribed, e g by the network or according to a standard, to betransmitted on the TFREs of the IMR. The receiving node 540 may in someembodiments be expected to measure interference on the IMR.

In step 930 the transmitting node 560 transmits, as said interference,at least one interfering signal on the IMR. The transmitted at least oneinterfering signal thereby causes the interference that the transmittingnode 560 is expected to transmit on the IMR. The at least oneinterfering signal comprises at least one of a desired signal andanother signal. The desired signal is expected to be decoded orcoherently measured upon by the receiving node 540 or another node 550served by said transmitting node 560. The desired signal may replace theanother signal on one or more TFREs of the IMR when the desired signalis present. In other words, the desired signal may be transmitted inplace of the another signal as said at least one interfering signal onone or more TFREs of the IMR when the desired signal is present at theat least one transmission point to be transmitted to the receiving node540 or to the another node 550 served by said transmitting node 560.When the desired signal is not present at the at least one transmissionpoint, or when no desired signal is present to be transmitted at the atleast one transmission point, the another signal is transmitted by theat least one transmission point as said at least one interfering signalon the one or more TFREs of the IMR. The another signal is not expectedto be decoded or coherently measured upon by any node 540, 550 served bythe transmitting node 560. In some embodiments, the another signal istransmitted on TFREs of said IMR where the desired signal is nottransmitted. Alternatively or additionally, the another signal may betransmitted on TFREs of said IMR where no desired signal is transmitted.The another signal is muted on TFRE's of said IMR where the desiredsignal is transmitted.

In an alternative, in step 910 the transmitting node 560 determines aninterference measurement resource, IMR. The IMR comprises set ofTime-Frequency Resource Elements, TFREs, upon which the transmittingnode is expected to transmit interference. No signal that is expected tobe decoded or coherently measured upon by any node 540, 550 served bythe transmitting node 560 is pre-scribed to be transmitted on the TFREs.In other words, a desired signal, that is expected to be decoded orcoherently measured upon by at least one of the receiving nodes 540, 550served by said transmitting node 560, is not pre-scribed, e g by thenetwork or according to a standard, to be transmitted on the TFREs ofthe IMR. The receiving node 540 may in some embodiments be expected tomeasure interference on the IMR.

According to the alternative, in step 930 the transmitting node 560transmits at least one interfering signal on the IMR. The at least oneinterfering signal comprises at least one of a desired signal andanother signal. The desired signal is expected to be decoded orcoherently measured upon by the receiving node 540 or another node 550served by said transmitting node 560. The another signal is not expectedto be decoded or coherently measured upon by any node 540, 550 served bythe transmitting node 560. In some embodiments, the another signal istransmitted on TFREs of said IMR where the desired signal is nottransmitted. Alternatively or additionally, the another signal may betransmitted on TFREs of said IMR where no desired signal is transmitted.The another signal is muted on TFRE's of said IMR where the desiredsignal is transmitted.

The transmitting node 560 may in at least some embodiments be configuredto select, momentarily for a subframe to be transmitted, whether totransmit the desired signal or the another signal on the one or moreTFREs of the IMR.

The desired signal may in at least some embodiments be a signal that isexpected to be decoded by the receiving node 540 or another node 550served by said transmitting node 560. In one example according to suchembodiments, the desired signal is a data signal transmitted on thePhysical Downlink Shared Channel.

In some embodiments, the at least one transmission point 510, 520, 530may comprise a composition of transmission points. The transmission ofat least one interfering signal in step 930 may then comprisetransmitting a respective interfering signal on the IMR from eachtransmission point in the composition of transmission points. Thecomposition of transmission points may be selected so that it enablesthe receiving node 540 to measure interference applicable to at leastone transmission hypothesis for which the receiving node 540 is toreport Channel State Information, CSI, to the transmitting node 560. Theselection of composition of transmission points may be made by thetransmitting node 560. Alternatively in some embodiments, the selectionmay be made by a controlling node that coordinates transmissions fromthe at least one transmission point 510, 520, 530 with transmissionsfrom at least one further transmission point controlled by a furthertransmitting node in the wireless communication system 500, 600, 700.

FIG. 9 b depicts an embodiment wherein the transmitting node 560 inaddition to performing steps 910 and 930 as above instructs thereceiving node 540, in a step 920, to measure interference on said IMR.Alternatively or additionally, the receiving node may be arranged tomeasure interference on said IMR without being explicitly instructed todo so by the transmitting node. In one particular embodiment, thereceiving node may be pre-configured to measure interference on saidIMR.

FIG. 9 c depicts an embodiment wherein the transmitting node 560 inaddition to performing steps according to any of the embodiments of FIG.9 a or 9 b determines, in a step 925, an expected spatial signature ofan intended data transmission to the receiving node 540 and, in a step928, applies said expected spatial signature when transmitting said atleast one interfering signal. The expected spatial signature may in someembodiments be determined based on respective buffer status and historicCSI reports of the receiving node 540 and at least one further receivingnode 550 served by the transmitting node 560.

FIG. 9 d shows an embodiment where the interference measurementsperformed according to any of the embodiments of FIG. 9 a, 9 b or 9 care used for link adaptation of a communication link between thetransmitting node 560 and the receiving node 540. The link adaptation isperformed as follows:

In step 940 the transmitting node 560 receives a Channel StateInformation, CSI, report from the receiving node 540. The CSI report isbased at least in part on interference measurements performed by thereceiving node 540 on the IMR.

In step 950 the transmitting node 560 transmits a data or control signalto the receiving node 540 using a resource allocation and/or linkadaptation that is based, at least in part, on the CSI report.

The IMR may in any one of the embodiments presented above be configuredto coincide with Time-Frequency Resource Elements, TFREs, of a CellSpecific Reference Signal, CRS, configuration. In some of theseembodiments, no CRS is transmitted from the at least one transmissionpoint 520, 530 on the TFREs of the CRS configuration. In theseembodiments, CRS may transmitted from a neighbouring transmission point510 on the TFREs of the CRS configuration and transmissions from the atleast one transmission point 520, 530 may be coordinated withtransmissions from the neighbouring transmission point 510. In someembodiments Coordinated Multipoint Transmission may ve applied tocoordinate the transmissions.

Alternatively or additionally, the IMR may in any one of the embodimentspresented above be configured to be covered by, overlap with or coincidewith Time-Frequency Resource Elements, TFREs, of a Channel StateInformation-Reference Signal, CSI-RS, configuration applied for thereceiving node 540. The impact of the CSI-RS may then need to becancelled out by the receiving node 540 when performing the interferencemeasurement.

Alternatively or additionally, the IMR may in any one of the embodimentspresented above be configured to be covered by, overlap with or coincidewith Time-Frequency Resource Elements, TFREs, of a zero power ChannelState Information-Reference Signal, CSI-RS, configuration applied forthe receiving node 540.

Furthermore, in any one of the embodiments presented above, a furtherdesired signal expected to be decoded or coherently measured upon by anyof the nodes 540, 550 served by the transmitting node 560 may betransmitted on at least one Time-Frequency Resource Element, TFRE, thatis covered by, covers, overlaps with or coincides with the IMR, inaddition to the transmitted at least one interfering signal. The impactof the further desired signal may then need to be cancelled out by thereceiving node 540 when performing the interference measurement.

Furthermore, in any one of the embodiments presented above, thetransmitted at least one interfering signal may be an isotropic signal.

Furthermore, in any one of the embodiments presented above, the IMRdetermined in step 910 may be a subset of a set of TFREs upon which saidreceiving node 540 is configured not to expect data transmissions fromany one of said at least one transmission point 510, 520, 530. In someof these embodiments, the set of TFREs may be a set of zero powerChannel State Information-Reference Signal, CSI-RS, configurations.

FIGS. 10 a-b illustrate devices configured to execute the methodsdescribed above in relation to FIG. 9 a-d.

FIG. 10 a illustrates a transmitting node 560, 1000 for enabling areceiving node 540 to perform measurements on interference. Thetransmitting node 560, 1000 enables the receiving node to performmeasurements on interference by transmitting an interfering signal. Thetransmitting node 560, 1000 is configured to be connectable to radiocircuitry 1010 comprised in at least one transmission point 510, 520,530 for communicating with the receiving node 540 in the wirelesscommunications system 500, 600, 700. Receptions at the receiving node540 may be susceptible to interference caused by transmissions from theat least one transmission point 510, 520, 530.

The transmitting node 560, 1000 comprises processing circuitry 1020. Theprocessing circuitry 1020 is configured to determine an interferencemeasurement resource, IMR. The IMR comprises set of Time-FrequencyResource Elements, TFREs, upon which the transmitting node is expectedto transmit interference. The receiving node 540 may in some embodimentsbe expected to measure interference on the IMR. The processing circuitry1020 is further configured to transmit as said interference, via the atleast one transmission point 510, 520, 530, at least one interferingsignal on said IMR. The transmitted at least one interfering signalthereby causes the interference that the transmitting node 560, 1000 isexpected to transmit on the IMR. The at least one interfering signalcomprises at least one of a desired signal and another signal. Thedesired signal is expected to be decoded or coherently measured upon bythe receiving node 540 or another node 550 served by said transmittingnode 560, 1000. The transmitting node 560, 1000 may be configured toreplace the another signal by the desired signal on one or more TFREs ofthe IMR when the desired signal is present. In other words, thetransmitting node 560, 1000 may be configured to transmit the desiredsignal in place of the another signal as said at least one interferingsignal on one or more TFREs of the IMR when the desired signal ispresent at the at least one transmission point 510, 520, 530 to betransmitted to the receiving node 540 or to the another node 550 servedby said transmitting node 560. When the desired signal is not present atthe at least one transmission point 510, 520, 530, or when no desiredsignal is present to be transmitted at the at least one transmissionpoint 510, 520, 530, the another signal is transmitted by the at leastone transmission point as said at least one interfering signal on theone or more TFREs of the IMR. The another signal is not expected to bedecoded or coherently measured upon by any node 540, 550 served by saidtransmitting node 560, 1000. In some embodiments, the processingcircuitry 1020 is configured to transmit the another signal on TFREs ofsaid IMR where the desired signal is not transmitted. Alternatively oradditionally, the processing circuitry 1020 may be configured totransmit the another signal on TFREs of said IMR where no desired signalis transmitted. The processing circuitry 1020 is configured to mute theanother signal on TFRE's of said IMR where the desired signal istransmitted.

In an alternative, no signal that is expected to be decoded orcoherently measured upon by any node 540, 550 served by the transmittingnode 560, 1000 is pre-scribed to be transmitted on the TFREs of the IMR.In other words, a desired signal, that is expected to be decoded orcoherently measured upon by at least one of the receiving nodes 540, 550served by said transmitting node 560, is according to the alternative,not pre-scribed, e g by the network or according to a standard, to betransmitted on the TFREs of the IMR.

The transmitting node 560, 1000 may in at least some embodiments beconfigured to select, momentarily for a subframe to be transmitted,whether to transmit the desired signal or the another signal on one ormore TFREs of the IMR.

The desired signal may in at least some embodiments be a signal that isexpected to be decoded by the receiving node 540 or another node 550served by said transmitting node 560. In one example according to suchembodiments, the desired signal is a data signal transmitted on thePhysical Downlink Shared Channel.

The processing circuitry 1020 may in some embodiments further beconfigured to receive, via the at least one transmission point 510, 520,530, a Channel State Information, CSI, report from the receiving node540. The CSI report may be based at least in part on interferencemeasurements performed by the receiving node 540 on said IMR. Further,the processing circuitry 1020 may be configured to transmit, via the atleast one transmission point 510, 520, 530, a data or control signal tosaid receiving node 540 using a resource allocation and/or linkadaptation that is based, at least in part, on said CSI report.

The processing circuitry 1020 may in further embodiments further beconfigured to instruct, via the at least one transmission point 510,520, 530, the receiving node 540 to measure interference on said IMR.

FIG. 10 b illustrates details of a possible implementation of processingcircuitry 1020.

FIG. 11 a shows a receiving node 1100 that may perform measurements oninterference as enabled by the method performed in the transmitting node1000.

FIG. 11 b illustrates details of a possible implementation of processingcircuitry 1120 of the receiving node 1100.

In some embodiments, the method performed in an eNodeB involves

-   -   1) Determining for a first UE an interference measurement        resource for a specific interference measurement.    -   2) Transmitting at least one interfering signal, over an        effective channel, on time/frequency resource elements        associated with said specific interference measurement resource,        wherein said interfering signal is independent of data/control        signalling    -   3) Receiving from said first UE a CSI report (that is expected        to be) based, at least in part, on interference measurements on        said specific interference measurement resource    -   4) Transmitting a data or control signal, whose resource        allocation and/or corresponding link adaptation is based, at        least in part, on said CSI report.

Here, an effective channel includes the radio propagation channel aswell as receive and transmit antenna gains. Moreover, an effectivechannel may involve multiple receive and/or transmit antennas, and canfurther include any virtualization of the antennas (i.e., lineartransformations of signals to be transmitted on multiple transmitantennas). Hence a special case of step 2) above is to transmit thespecific interfering signal from all antennas of a specific transmissionpoint. Potentially, also a second interfering signal (on the same IMR)may be transmitted also from the antennas of a second transmissionpoint, and so on.

A special embodiment, where the IMR is interpreted as a CRS resource, isin a heterogeneous deployment where at least one pico node shares thesame cell ID as a macro node whose coverage area at least partiallyoverlaps with the pico node's coverage area. With this particulardeployment, the pico nodes and the macro will share the same CRSresource element positions (in fact also the same CRS sequence will beshared). Hence, with this configuration, the data transmissions isalways rate matched around (e.g., muted) on the TFREs associated withthe CRS, and thus the UEs served by the shared cell will not measure anyinterference from the nodes sharing the cell ID (presuming that theyperform the interference measurement on the CRS positions). Hence byactively transmit an interfering signal from at least one of thetransmission points (e.g., one of the pico nodes) on the CRS TFREs a Rel8-10 LTE terminal will report CSI corresponding to present interferencefrom said at least one transmission points. This is particularlyinteresting if the CRS is only transmitted from the macro node, whereasthe pico nodes mute the CRS, and act purely as performance boosters forRel-11 and beyond terminals, using transmission modes not relying onCRS. Hence, Release 8-10 terminals that connect to the macro will still,due to the interfering signals transmitted from the pico nodes on theCRS positions, still measure relevant interference.

In a another embodiment the Determining in step 1) comprisesconfiguring, for example by means of radio resource control messages,said first UE to perform said specific interference measurement on aninterference measurement resource selected by the eNodeB.

This will be the typical implementation for LTE Rel 11 terminals andbeyond, where the IMR will be configurable.

In a further embodiment, the eNodeB configures also a second UE for datareceptions that are muted on said specific interference measurementresource.

This embodiment reflects that by configuring muting for all terminals onone, or the union of multiple, IMR(s) then the network can take fullcontrol of the interference seen on the IMR(s) independently of thescheduling and the data transmissions.

In one embodiment, said interfering signal is an isotropic signal. Inthis context an isotropic signal refers to a signal that excites alldimensions of an effective channel.

By transmitting an isotropic interference signal (i.e., a spatiallywhite, uncorrelated) signal no special bias to any spatial direction isimposed on the interference signal, which prevents the UE from makingparticular interference suppression assumptions of low rank interferencein the CSI reporting (e.g., in the interference suppression algorithms).

In another embodiment, the eNodeB further determines an expected spatialsignature of a subsequent data transmission, and applies said expectedspatial signature on said interfering signal.

Such a spatial signature could be wideband or frequency selective. Ifthe eNodeB can predict the spatial characteristics (e.g., typicalbeamforming or precoding directions) for subsequent data transmissions,it is beneficial to impose the same characteristics on the interferingsignal, since this will allow a UE to better predict the actualinterference suppression performance in the CSI reporting, and thusenable more accurate link adaptation in the eNodeB.

In one such embodiment, the eNodeB determines said expected spatialsignature based on the buffer statuses of connected UEs and historic CSIreports from said connected UEs.

In a second such embodiment, the eNodeB excludes the non-primary (e.g.the non-strongest) transmission points participating in a jointtransmission to a UE from the spatial signature.

This embodiment ensures that the transmit signal from a non-servingtransmission point participating in a joint transmission is notaccounted for as interference in the subsequent CSI report from thetargeted UE.

In another embodiment, said interfering signal is constructed to havethe same spatial signature as a current (or past) data and/or controltransmission.

In one such embodiment, the interfering signal is muted on associatedresource elements in RBs where there is no current data assignments tobe transmitted over said effective channel.

This will effectively cause the UE to measure an interferencecorresponding to a data transmission, but without the restrictions ofcollisions with regular data.

Such embodiments can be beneficial when the scheduling of differenttransmission points are only partially coordinated; for example, whenspecific resource blocks (in time and/or frequency) are prioritized fortransmissions from a specific node (but may be used by other nodes whenthe resource need is sufficiently high). In such limited coordinationscenarios, there can typically not be a joint (central) scheduler thatbenefits from having on/off kind of interference estimates. Instead, itis typically more beneficial to let the interference measurement bebiased by the current load in the system. For example, one IMR could beconfigured to capture the interference from data corresponding toprioritized resource blocks, and another IMR could be configured tocapture the interference corresponding to the data interference onnon-prioritized RB.

In another embodiment, said interfering signal is constructed to havethe spatial signature that is a linear combination of the spatialsignatures of current and past data and/or control transmission.

In one such embodiment, the interfering signal is muted on associatedresource elements in RBs where there is no current data assignments tobe transmitted over said effective channel.

The invention provides a solution to freely construct the interferencecomposition on an IMR without any limitations imposed by UE specificmuting configurations. Moreover, the interference measurements can bemade to better reflect the performance when there is actualintra-cluster interference present without bias imposed by varyingtraffic load in the system.

This will translate to improved link adaptation and spectral efficiencyin the wireless system.

When using the word “comprise” or “comprising” it shall be interpretedas non-limiting, i.e. meaning “consist at least of”.

The present invention is not limited to the above-describe preferredembodiments. Various alternatives, modifications and equivalents may beused.

1. A method in a transmitting node for enabling a receiving node toperform measurements on interference caused by transmissions from atleast one transmission point controlled by the transmitting node onreceptions at the receiving node, the transmitting and receiving nodesbeing comprised in a wireless communications system, the methodCHARACTERIZED IN comprising: determining an interference measurementresource, IMR, comprising a set of Time-Frequency Resource Elements,TFREs, upon which the transmitting node is expected to transmitinterference; and transmitting at least one interfering signal on saidIMR as said interference, wherein the at least one interfering signalcomprises a desired signal, that is expected to be decoded or coherentlymeasured upon by the receiving node or another node served by saidtransmitting node, and another signal, that is not expected to bedecoded or coherently measured upon by any node served by saidtransmitting node, wherein the desired signal is transmitted in place ofthe another signal as said at least one interfering signal on one ormore TFREs of the IMR when the at least one transmission point is totransmit the desired signal to the receiving node or to the another nodeserved by said transmitting node and wherein the another signal istransmitted on TFREs of said IMR where no desired signal is transmittedand wherein the another signal is muted on TFRE's of said IMR where thedesired signal is transmitted.
 2. The method of claim 1, furthercomprising selecting, for a subframe to be transmitted, whether totransmit the desired signal or the another signal on the one or moreTFREs of the IMR.
 3. The method of claim 1, wherein desired signal isexpected to be decoded by the receiving node or another node served bysaid transmitting node.
 4. The method of claim 1, wherein the desiredsignal is a data signal transmitted on the Physical Downlink SharedChannel.
 5. The method of claim 1, wherein transmissions from differenttransmission points are coordinated in order to control interferenceand/or improve received signal quality in the wireless communicationssystem.
 6. The method of claim 1, wherein the interference measurementsare used for link adaptation of a communication link between thetransmitting node and the receiving node, the method comprising thefurther steps of: receiving a Channel State Information, CSI, reportfrom the receiving node, wherein the CSI report is based at least inpart on interference measurements performed by the receiving node onsaid IMR; transmitting a data or control signal to said receiving nodeusing a resource allocation and/or link adaptation that is based, atleast in part, on said CSI report.
 7. The method of claim 1, comprisingthe further step of: instructing the receiving node to measureinterference on said IMR.
 8. The method of claim 1, wherein the at leastone transmission point comprises a composition of transmission pointsand wherein transmitting the at least one interfering signal comprisestransmitting a respective interfering signal on said IMR from eachtransmission point in said composition of transmission points.
 9. Themethod of claim 1, wherein said composition of transmission points isselected so that it enables the receiving node to measure interferenceapplicable to at least one transmission hypothesis for which thereceiving node is to report Channel State Information, CSI, to thetransmitting node. 10-12. (canceled)
 13. The method of claim 1, whereinno CRS is transmitted from the at least one transmission point on theTFREs of the CRS configuration.
 14. The method of claim 1, wherein CRSis transmitted from a neighbouring transmission point on the TFREs ofthe CRS configuration and wherein transmissions from the at least onetransmission point are coordinated with transmissions from theneighbouring transmission point. 15-18. (canceled)
 19. The method ofclaim 1, further comprising the step of: determining an expected spatialsignature of an intended data transmission to the receiving node;applying said expected spatial signature when transmitting said at leastone interfering signal. 20-23. (canceled)
 24. A transmitting node forenabling a receiving node to perform measurements on interference, thetransmitting node being configured to be connectable to at least onetransmission point in a wireless communications system, whereinreceptions at the receiving node are susceptible to interference causedby transmissions from the at least one transmission point, thetransmitting node CHARACTERIZED IN comprising: processing circuitryconfigured to determine an interference measurement resource, IMR,comprising a set of Time-Frequency Resource Elements, TFREs, upon whichthe transmitting node is expected to transmit interference, theprocessing circuitry further configured to transmit as saidinterference, via the at least one transmission point, at least oneinterfering signal on said IMR, wherein the at least one interferingsignal comprises a desired signal, that is expected to be decoded orcoherently measured upon by the receiving node or another node served bysaid transmitting node, and another signal, that is not expected to bedecoded or coherently measured upon by any node served by saidtransmitting node, the processing circuitry further configured totransmit the desired signal in place of the another signal as said atleast one interfering signal on one or more TFREs of the IMR when the atleast one transmission point is to transmit the desired signal to thereceiving node or to the another node served by said transmitting node,the processing circuitry further configured to transmit the anothersignal on TFREs of said IMR where no desired signal is transmitted andto mute the another signal on TFRE's of said IMR where the desiredsignal is transmitted.
 25. The transmitting node of claim 24, furthercomprising selecting, for a subframe to be transmitted, whether totransmit the desired signal or the another signal on the one or moreTFREs of the IMR.
 26. The transmitting node of claim 24, wherein desiredsignal is expected to be decoded by the receiving node or another nodeserved by said transmitting node.
 27. The transmitting node of claim 24,wherein the desired signal is a data signal transmitted on the PhysicalDownlink Shared Channel.
 28. The transmitting node of claim 24, whereintransmissions from different transmission points are coordinated inorder to control interference and/or improve received signal quality inthe wireless communications system.
 29. The transmitting node of claim24, wherein the transmitting node is configured to use the interferencemeasurements for link adaptation of a communication link between thetransmitting node and the receiving node, the processing circuitryfurther configured to: receive, via the at least one transmission point,a Channel State Information, CSI, report from the receiving node,wherein the CSI report is based at least in part on interferencemeasurements performed by the receiving node on said IMR; and transmit,via the at least one transmission point, a data or control signal tosaid receiving node using a resource allocation and/or link adaptationthat is based, at least in part, on said CSI report.
 30. Thetransmitting node of claim 24 wherein the processing circuitry isfurther configured to: instruct, via the at least one transmissionpoint, the receiving node to measure interference on said IMR.
 31. Thetransmitting node of claim 24, wherein the at least one transmissionpoint comprises a composition of transmission points and wherein theprocessing circuitry is configured to transmit the at least oneinterfering signal by transmitting a respective interfering signal onsaid IMR via each transmission point in said composition of transmissionpoints.
 32. The transmitting node of claim 24, wherein the processingcircuitry is configured to select said composition of transmissionpoints so that it enables the receiving node to measure interferenceapplicable to at least one transmission hypothesis for which thereceiving node is to report Channel State Information, CSI, to thetransmitting node. 33-35. (canceled)
 36. The transmitting node claim 35,wherein the processing circuitry is configured to coordinatetransmissions via the at least one transmission point with transmissionsfrom a neighbouring transmission point from which CRS is transmitted onthe TFREs of the CRS configuration. 37-40. (canceled)
 41. Thetransmitting node of claim 24, wherein the processing circuitry isfurther configured to determine an expected spatial signature of anintended data transmission to the receiving node and to apply saidexpected spatial signature when transmitting said at least oneinterfering signal. 42-45. (canceled)